In the field of hydrocarbon exploration and production, various tools are used to provide fluid seals between two components in a wellbore. Annular barriers have been designed for preventing undesirable flow of wellbore fluids in the annulus between a wellbore tubular and the inner surface of a surrounding tubular or the borehole wall. In many cases, the annular barriers provide a fluid seal capable of holding a significant pressure differential across its length. In one application, a wellbore packer is formed on the outer surface of a completion string which is run into an outer casing in a first condition having a particular outer diameter. When the packer is in its desired downhole location, it is inflated or expanded into contact with the inner surface of the outer casing to create a seal in the annulus. Similar wellbore packers have been designed for use in openhole environments, to create a seal between a tubular and the surrounding wall of the wellbore.
Conventional packers are actuated by mechanical or hydraulic systems. A force or pressure is applied from surface to radially move a mechanical packer element into contact with the surrounding surface. In an inflatable packer, fluid is delivered from surface to inflate a chamber defined by a bladder around the tubular body.
More recently, wellbore packers have been developed which include a mantle of swellable material formed around the tubular. The swellable material is selected to increase in volume on exposure to at least one predetermined fluid, which may be a hydrocarbon fluid or an aqueous fluid or brine. The swellable packer may be run to a downhole location in its unexpanded state, where it is exposed to a wellbore fluid and caused to increase in volume. The design, dimensions and swelling characteristics are selected such that the swellable packer element expands to create a fluid seal in the annulus to isolate one wellbore section from another. Swellable packers have several advantages over conventional packers, including passive actuation, simplicity of construction, and robustness in long term isolation applications.
In addition, swellable packers may be designed for compliant expansion of the swellable mantle into contact with a surrounding surface, such that the force imparted on the surface prevents damage to a rock formation or sandface, while still creating an annular barrier or seal. Swellable packers therefore lend themselves well to openhole completions in loose or weak formations.
The materials selected to form a swellable element in a swellable packer vary depending on the specific application. Swellable materials are elastomeric (i.e. they display mechanical and physical properties of an elastomer or natural rubber). Where the swellable mantle is designed to swell in hydrocarbons, it may comprise a material such as an ethylene propylene diene monomer (EPDM) rubber. Where the swellable mantle is required to swell in aqueous fluids or brines, the material may for example comprise an N-vinyl carboxylic acid amide-based crosslinked resin and a water swellable urethane in an ethylene propylene rubber matrix. Suitable materials for swellable packers are described in GB 2411918 or WO2005/012686. In addition, swellable elastomeric materials designed to increase in volume in both hydrocarbon fluids and aqueous fluids are described in the applicant's co-pending International patent publication numbers WO2008/155564 and WO2008/155565.
Applications of swellable tools are limited by a number of factors including their capacity for increasing in volume, their ability to create a seal, and their mechanical and physical properties when in their unexpanded and expanded states. A swellable packer may be exposed to high pressure differentials during use. The integrity of the annular seal created by a well packer is paramount, and a tendency of the swellable material to extrude, deform or flow under forces created by the pressure differential results in a potential failure mode between the apparatus and the surrounding surface. In practice therefore, swellable tools and in particular swellable packers, will be designed to take account of the limitations of the material. For example, a swellable packer may be run with an outer diameter only slightly smaller than the inner diameter of the surrounding surface, in order to limit the percentage volume increase of the swellable material during expansion. In addition, swellable packers may be formed with packer elements of significant length, greater than those of equivalent mechanical or hydraulic isolation tools, in order to increase the pressure rating and/or reduce the chances of breaching the seal at high differential pressures.
International patent publication number WO 2006/121340 describes an expandable end ring for a swellable packer which is said to anchor the packer material to the tubular more effectively. However, the arrangement of WO 2006/121340 does not address the problems of extrusion of the swellable material in use.
The applicant's co-pending International patent publication number WO 2008/062186 describes a support structure suitable for use with a swellable packer, which is operable to be deployed from a first unexpanded condition to a second expanded condition by the swelling of the packer. By providing a support structure which substantially covers the end of the swellable mantle, extrusion of the swellable material is mitigated. This permits packers to be produced with a required pressure rating which are shorter in length than conventional swellable packers. Furthermore, packers can be formed with reduced outer diameter, as the mechanical strength of the elastomeric material is less critical. The packer can therefore be engineered to have a larger expansion factor while maintaining shear strength and differential pressure rating. The arrangement of WO 2008/062186 therefore allows a swellable packer to be used over a wider range of operating parameters. Although the arrangement of WO 2008/062186 is suitable for use in many wellbore applications, in certain conditions its effectiveness and/or practicality are limited.
It is one aim of an aspect of the invention to provide a support assembly for a swellable material in a downhole apparatus, which is improved with respect to previously proposed support assemblies.
Other aims and objects will become apparent from reading the following description.